What the valve is for
Every tank water heater has a temperature and pressure relief valve, usually called the T&P valve, near the top of the tank with a pipe running down or out from it. If the water inside gets too hot or the pressure climbs too high, the valve opens and releases water so the tank cannot fail violently.
That makes a dripping relief valve different from an ordinary leak. Replacing the valve without asking why it opened can hide a pressure or temperature problem that will come back.
Likely causes
| Cause | What it looks like | Usual fix |
|---|---|---|
| High incoming water pressure | Drips at many times of day; faucets and toilets may also run or bang | Test pressure; install or adjust a pressure regulator |
| Thermal expansion in a closed system | Drips or spurts while the heater is heating, often after a regulator or check valve was added | An expansion tank sized for the system, if your plumber recommends one |
| Thermostat set too high or failing | Very hot water at the taps and discharge during heating | Lower the setting or replace the thermostat or gas control |
| Worn or debris-fouled valve | Slow constant drip with normal pressure and temperature | Replace the relief valve with the correct rating |
How the cause is usually narrowed down
- Measure static pressure.A gauge on a hose bib or laundry valve shows the pressure coming into the house. Readings are compared against the plumbing code limit for homes.
- Watch pressure during a heating cycle.If pressure climbs while the water heats and nothing is running, thermal expansion is the likely culprit.
- Check water temperature at a tap.Scalding water points to a thermostat or control problem rather than pressure.
- Inspect the valve and discharge line.The line should be the right size, run to an approved location, and not be blocked or threaded closed at the end.
Terms you may hear
- T&P valve
- Temperature and pressure relief valve. Opens if tank temperature or pressure exceeds its rating.
- Pressure regulator (PRV)
- A valve near where the main line enters the house that lowers incoming city pressure to a set level.
- Thermal expansion
- Water expands as it heats. In a closed system the extra volume has nowhere to go, so pressure rises.
- Expansion tank
- A small tank with an air cushion, usually on the cold line near the heater, that absorbs expanding water.
- Closed system
- A plumbing system where a check valve or regulator stops water from flowing back toward the street.
Questions to ask
- What is my static water pressure, and what does the code allow?
- Is my system closed, and does it need an expansion tank?
- Is the relief valve itself worn, or is it reacting to something else?
- Is the discharge pipe installed and terminated correctly?
Common questions
Can I just replace the relief valve myself?
The valve must match the heater's rating and be installed correctly with a proper discharge line. Because a dripping valve often signals a pressure problem, it is worth having a licensed plumber diagnose it first.
Why did the dripping start after other plumbing work?
Adding a pressure regulator or a check valve can turn an open system into a closed one, which makes thermal expansion show up at the relief valve. Mention any recent work to your plumber.
Is a small drip dangerous?
The drip itself is the safety device working. The risk is ignoring it, or blocking it, while high pressure or temperature continues.
General information for Golden Hills, not a quote or professional advice. In an emergency, call 911.